1. Field of the Invention
The invention relates to forced draft water heaters or boilers and particularly to a forced draft water heater or boiler providing efficient water heating at all demand levels, substantially without interruption of burner operation.
2. Description of the Prior Art
Water heaters or boilers employing forced draft burners have used control systems to respond to variations in exogenous demand for hot water from the water heater or boiler. U.S. Pat. No. 4,519,540 to Boulle et al., teaches a water boiler in which heat input to the water boiler is adjusted for exogenous demand, while taking into account various environmental factors and user history. The variables controlled to control heat input are gas flow to a burner and speed of a fan providing the forced air flow. Gas flow and air flow into a combustion chamber are adjusted with reference to one another to produce complete burning without excess air being drawn. Excess air flow results in higher than required motor loads and the need to heat greater quantities of air than needed to meet burner demand.
Boulle et al., however, define demand for heating of the water in terms of an order from a consumer, i.e. exogenous demand. When no exogenous demand exists, circulation of hot water is eliminated, fan or blower speed is reduced to 50% of nominal and the burners (except for a pilot) are turned off. The continuing flow of air from the blower, however through the combustion chamber will extract heat from the boiler.
Periodic or occasional shutdown of burners in large, commercial scale water heaters and boilers requires a purge of the combustion chamber after heating is discontinued and before it resumes. A purge is acceleration of air flow through the combustion chamber with no gas flow, done to assure no stray gas is left in the chamber when combustion resumes. If any gas is left in the chamber, resumed burning may result in ignition of the stray gas in an undesired portion of the combustion chamber or in an exhaust stack. Some potential for explosion exists under such conditions. However, purging of a combustion chamber by forcing unheated air through the chamber may be thermally wasteful.